1. Statement of the Technical Field
The present invention relates to the field of electromagnetic horn antennas, and more particularly to horn antennas that use dielectric loading.
2. Description of the Related Art
Electromagnetic horn antennas are commonly used to produce a directional RF radiation pattern at microwave frequencies. A horn antenna generally includes a conical or rectangular wall section for transmitting and/or receiving an electromagnetic signal. The wall flares or angles outwardly from a throat section to an aperture and defines an internal surface made out of electrically conductive material. The throat of the microwave horn is typically sized to be comparable to the wavelength being used. Horn radiators may be fed by waveguides, coaxial lines and other feeds.
A horn antenna is an electromagnetic transducer which gradually transforms the wave impedance at the throat of the horn to the impedance of free space at the aperture end. A horn antenna can be viewed as an “improved” waveguide radiator. The simplest waveguide radiator is an open-ended waveguide. The directivity of a waveguide radiator may be increased by enlarging the aperture. This is done by attaching a flare or horn to the waveguide, hence the term tapered horn antenna. The tapered horn antenna is designed to transform a transverse wave at the end of the waveguide to a similar transverse wave at the end of the tapered horn without causing attenuation. The throat of the tapered horn (the junction between the tapered horn and the waveguide) serves as a filter device and allows only a single mode to be propagated freely to the aperture. The tapered horn will not support propagation of a particular mode unless the transverse dimensions of the tapered horn are greater than the dimensions of the waveguide.
The dimensions of the open end of the tapered horn are chosen to obtain the desired radiation pattern and to prevent spherical distortion of the propagated wave. The taper of the horn serves to match the impedance of the waveguide to the impedance of space. At one end, the impedance of the tapered section matches that of space; at the other end, it matches the impedance of the waveguide.
Electromagnetic horn antennas are available in several different configurations including rectangular, pyramidal, and conical designs. The radiating field pattern of the horn will generally be determined by the shape that is selected. Horns with larger mouths tend to have more directive field patterns. The flare angle of the horn largely determines the phase distribution of the fields at the mouth of the horn, which influences the distribution and level of sidelobes in the far field radiation pattern. In general, small flare angles produce the least phase variation and the most desirable patterns. However, the combination of a large mouth and small flare angle leads to a long horn. Horn design requires balancing these parameters against the physical constraints of the application. Structures such as metallic septa or stepped dielectric slabs can also be placed within the horn to change the wave velocity across the horn and thus control phase distribution at the aperture.
One advantage of conventional horn antennas is that they generally will operate reasonably well over a relatively broad range of frequencies. However, increasing demands for wideband and multi-band operational capability have placed even more emphasis on the need for expanding the range of frequencies over which a single horn antenna can be made to operate.